Cold Big Bang Nucleogenesis

This paper treats the energetic and nucleosynthetic history of a Friedmann model with low photon-baryon ratio ηγ and positive lepton-baryon ratio ηL. At early times, such a universe is in equilibrium and is energetically dominated by particle Fermi energies; at later times photon release from elementary particle decays probably lifts degeneracy of nucleons but not of leptons. After outlining the early history I present the results of full nucleosynthesis calculations for nondegenerate baryons and degenerate electrons and neutrinos. The results show that, for ηγ ≲ 0.01 (consistent with the particle-decay heating), the neutron-proton ratio depends strongly on ηL and that production of sufficient helium to match observations creates large primordial metallicity, in agreement with a previous argument by Carr for the ηγ = 0 case. Excessive metallicity in the cold model is avoided only if ηL is sufficiently high (ηL > 1.5 for ηγ = 0; ηL ≳ 5 for ηγ = 0.01) to suppress all nucleosynthesis before the Population III epoch. The calculations also reveal certain combinations of ηγ ≳ 100 (corresponding to T0 ~ 0.02) and ηL ≳ 5 that produce acceptable helium yields. This result contradicts the widespread notion that astrophysical observations of helium abundance together with nucleosynthesis calculations within a big bang model predict the current radiation background temperature.